1. Field of the Invention
This invention relates to a tablet coating apparatus, more particularly, a tablet coating apparatus for coating pharmaceutical or food tablets or the like with a layer of other predetermined medical composition or sugar etc., which apparatus includes a rotatable coating pan or drum, the side wall of the drum has a ventilating area for feeding drying air.
2. Prior Art
Apparatus generally employed in the tablet coating art of this kind comprises a cylindrical container or drum mounted for rotary movement for the purpose of tumbling the tablets charged therein. The coating material in solution or suspension is introduced into the drum by a conventional means such as hydraulic spraying equipment. As the tablets tumble in the drum, they become coated with the fluid coating material. The coating or coated layer thus applied to tablets has to be thereafter dried to be hardened or solidified.
In a prior tablet coating apparatus, in which tablet or other granular raw material is sprayed with coating materials such as medical or sugar solution etc. and then the coated material is dried, heated air is blown onto the surface of the bed of tablets while the tablets to be coated are tumbled in a coating drum by rotating the drum. However, in this prior apparatus, although it is possible to dry only the tablets in the surface zone of the bed, the tablets in the inside and lower zones of the bed are insufficiently dried because the heated air can not reach to such zones. Therefore, it takes very long time to dry all of the tablets in the drum, and it is difficult to dry the tablets uniformly.
For eliminating these drawbacks, the applicant has provided a coating apparatus shown in FIGS. 1 and 2 which are disclosed in the Japanese Patent Publication No. 50-38713. This coating apparatus is provided with a ventilating and drying mechanism comprised of perforated areas 2 for ventilating air arranged axially in a plurality of circumferential positions or whole circumference of a rotary container or drum 1, an air supply conduit 3, an exhaust or suction duct 4, and an exhaust duct 5.
In this coating apparatus, the hot air blown into the drum 1 from the air supply conduit 3 is blown onto tablets 6 tumbled in the drum 1, and after passing through the bed of the tablets 6, the hot air is exhausted from the exhaust conduit 5 via the perforated area 2 and the duct 4.
Therefore, this coating apparatus is used as an excellent coating apparatus in a very wide range of field, because, in this coating apparatus, the hot air can contact uniformly with the tablets 6 not only in the surface zone but also in the inside and lower zones of the bed thereof, the heat efficiency is very high, it is possible to obtain homogeneous product dried with overall uniformity and with high productivity. Further, as the result of the high heat efficiency provided by this coating apparatus, it is possible to perform coating operation efficiently and in low cost, using an aqueous solution wherein coating material is dissolved or dispersed in the water (in this connection, refer to the Japanese Patent Publication No. 55-5491).
Moreover, in FIGS. 1 and 2, the reference numeral 7 is a rotary shaft for rotating the drum 1, 8 is a motor for driving the rotary shaft 7 via transmission means 9 such as a belt or a chain, 10 is a cylindrical portion which forms a charging opening through which tablets 6 to be coated are charged into the drum 1, and 11 is a cover for closing and opening the open end of the cylindrical portion 10. These cylindrical portion 10, cover 11 and said air supply conduit 3, exhaust conduit 5 do not rotate. The connecting portions between these non-rotating members and rotating members of the drum 1 are sealed by labyrinth type sealing means etc.
In such a coating apparatus, there are provided a plurality of ventilating through apertures having circular, rectangular or oblong or ellipse shapes in the perforated area 2 to increase the numerical aperture of the perforated area 2 in order to reduce the resistance to the ventilation through the perforated area 2. But, as a large quantity of coating materials are processed in the drum, the tablets or other granular materials being coated are clogged in the ventilating apertures to block the apertures. When the ventilating apertures are blocked, the smooth flow of the hot air is prevented to produce non-uniform ventilation, and the pressure loss of the ventilation system becomes increased or changed to make the coating operation instable. Further, it takes longer time for drying because of the reduction of the heat efficiency.
Alternatively, there is proposed other coating system, in which powdery raw material is coated in place of a portion of coating solution to reduce the energy for drying by using the above described coating apparatus. However, in this coating system, because the powdery material is leaked through the ventilating aperture and the leaked powdery material is escaped out of the system through the exhaust duct and conduit, not only a product having non-uniform components is produced, but also loss of expensive powdery raw material and environmental pollution occur.
Such problems arise not only in a conical shaped coating apparatus, but also in other coating apparatus, for example, a cylindrical coating apparatus disclosed in the Japanese Utility Model Publication No. 56-21466, or an onion or a pear shaped coating apparatus.